Metal surfaces coated with fluorinated polymers

ABSTRACT

The present invention relates to a coated metal surface comprising, successively: 
     a layer (1) of epoxy primer placed next to the metal, 
     a layer (2) of binder comprising 98 to 50 parts by weight of at least one fluoropolymer L3 per 2 to 50 parts, respectively, of at least one polymer chosen from acrylic polymers L1 and polymers L2 which are fluoropolymers chemically modified by a partial dehydrofluorination followed by an oxidation, 
     a layer (3) of fluoropolymer. 
     According to a first variant, the coating does not comprise the layer (3). However, it is recommended that the layer (2) which becomes the outer layer should be thicker than in the structure of the main invention. 
     According to a second variant, the coating does not comprise the layer of primer (1), the layer of binder necessarily contains the polymer L2 and the surface is necessarily the outer surface of tubes. 
     According to a third variant, the coating does not comprise the layer (2) and the layer (1) comprises a mixture of epoxy primer and polymer L2. 
     The invention relates more particularly to the coating of the outer surface of tubes. These tubes are useful for the development of offshore hot oil wells, since it is necessary for the tubes which transport the hot oil to withstand corrosion by seawater.

FIELD OF THE INVENTION

The present invention relates to metal surfaces coated withfluoropolymers and more particularly to tubes whose outer surface iscoated with fluoropolymers. These tubes are useful for the developmentof offshore hot oil wells, since it is necessary for the tubes whichtransport hot oil to withstand corrosion by seawater.

THE TECHNICAL PROBLEM

No steel tube coating which can be readily produced industrially andwhich can withstand high temperature under offshore conditions is knownat the present time. For the development of offshore hot wells, onesolution consists in cooling the oil with a heat exchanger beforeraising it to the surface. This technique is very expensive.Furthermore, the cooling may result in the formation of a cold plug. Itis also possible to make use of special steels, but they areprohibitively expensive. A coating made of fluoropolymer, for examplePVDF (common abbreviation for polyvinylidene fluoride), as disclosed inthe invention makes it possible to convey hot fluids (150° C.) underoffshore conditions, using ordinary steel tubes.

PRIOR ART

Patent DE 3 422 920 discloses outer coatings for steel tubes,comprising, successively, a layer of epoxy resin, a layer of graftedpolypropylene and, finally, an outer layer of a mixture of polypropyleneand of a polypropylene/polyethylene block copolymer. The glasstransition temperature Tg of the epoxy resin is between 80° C. and 94°C. These coatings are suitable for hot water at 90° C.

U.S. Pat. No. Re 30 006 discloses outer coatings for steel tubes,comprising, successively, an epoxy resin and a polyethylene modifed bygrafting or copolymerization with maleic anhydride. Nothing is statedregarding the Tg of the epoxy resin; however, the polyethylene does notmake it possible to work above 80° C.

Patent EP-A-0 770 429 discloses a coated metal surface such as the outersurface of a tube, comprising, successively, a layer of epoxy resinplaced next to the metal and having a glass transition temperature ofgreater than 120° C., a layer of binder based on polypropylene modifiedby grafting and a layer of thermoplastic polymer. The thermoplasticpolymer is chosen from polyamides, polyamide alloys and polypropylene.These coatings do not offer any protection against corrosion by seawaterto tubes transporting oil at 150° C.

Patent EP-A-404 752 discloses structures consisting, successively, of asubstrate, a primer and a layer of PVDF. The primer is a mixture of anepoxy resin with either PMMA (common abbreviation for polymethylmethacrylate) or a copolymer of methyl methacrylate and of ethylacrylate. Patent EP-A-0 354 822 discloses similar structures. Thiscoating does not withstand corrosion by seawater when the tubestransport oil at 150° C.

Patent WO 97/27260 discloses structures consisting, successively, of asubstrate, a primer and a layer of PVDF. The primer is a mixture of atleast two of the following three polymers, namely (i) a PVDFhomopolymer, (ii) a PVDF copolymer comprising at least 50 mol % of VF2and (iii) an acrylic polymer containing carboxylic acid functions, suchas, for example, copolymers of methyl methacrylate and of acrylic acid.The substrate may be the outer surface of a tube. Patent WO 97/49777discloses similar structures. This coating does not withstand corrosionby seawater when the tubes transport oil at 150° C.

It has now been found that a coating comprising, respectively, a layerof epoxy resin, a layer of binder based on PVDF and on at least onepolymer chosen from acrylic polymers and oxidized fluoropolymers and alayer of PVDF, the epoxy resin being on the side of the metal, protectsthe metal against corrosion by seawater even if the metal is at 150° C.Variants of this coating, which are detailed later, may also be used.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a coated metal surface comprising,successively:

a layer (1) of epoxy primer placed next to the metal,

a layer (2) of binder comprising 98 to 50 parts by weight of at leastone fluoropolymer L3 per 2 to 50 parts, respectively, of at least onepolymer chosen from acrylic polymers L1 and polymers L2 which arefluoropolymers chemically modified by a partial dehydrofluorinationfollowed by an oxidation,

a layer (3) of fluoropolymer.

The present invention also relates to a process for manufacturing thesecoated surfaces. The metal surface is first degreased, sanded and thenheated. The epoxy primer of the layer (1) is deposited in liquid form orby spraying or electrostatic spraying if it is a powder onto the metalsurface heated to 200-240° C. After about 20 to 30 seconds, i.e.slightly before the end of the gel time and before the resin hascrosslinked, in order for there to remain epoxide functions to reactwith the binder, the binder of the layer (2) is deposited either byspraying if it is in powder form, or by coating or rolling. Next, thefluoropolymer of the layer (3) is deposited in the same way. As regardsthe outer surface of metal tubes, the process is performed in the sameway for the epoxy primer, and the binder is then either deposited byspraying if it is available in powder form, or extruded through acircular die (also known as a crosshead) arranged concentrically aroundthe tube. The binder may also be extruded through a flat die producing acontinuous strip which is wound around the tube, for example by means ofrotating the tube on itself. The fluoropolymer is deposited in the sameway.

The coating of the invention can readily be produced on a conventionalcoating line, due to the excellent processability of PVDF. The coatingcan be applied continuously, at a speed of at least 50 cm/minute, at atemperature of less than 250° C., this temperature making it possible toconserve all the initial properties of the steel. The ease of use is anadvantage over other known solutions thermostable polymers such aspolysulfone, polyphenylene ether or polyether imide which require eitherhigh temperatures or a long and intricate implementation with reactivesolvents (post-curing).

According to a first variant, the coating does not comprise the layer(3). However, it is recommended that the layer (2) which becomes theouter layer should be thicker than in the structure of the maininvention.

According to a second variant, the coating does not comprise the layerof primer (1), the layer of binder necessarily contains the polymer L2and the surface is necessarily the outer surface of tubes.

That is to say that in this second variant, the invention relates to acoated metal surface which is the outer surface of tubes, comprising,successively:

a layer (2) of binder placed next to the metal and comprising 98 to 50parts by weight of at least one fluoropolymer L3 per 2 to 50 parts,respectively, of a mixture comprising at least one polymer chosen frompolymers L2 which are fluoropolymers chemically modified by a partialdehydrofluorination followed by an oxidation, and optionally at leastone polymer chosen from acrylic polymers L1,

a layer (3) of fluoropolymer.

According to a third variant, the coating does not comprise the layer(2) and the layer (1) comprises a mixture of epoxy primer and of polymerL2.

That is to say that in this third variant, the invention relates to acoated metal surface comprising, successively:

a layer (1) of primer placed next to the metal and comprising 1 to 70parts of a polymer chosen from polymers L2 which are fluoropolymerschemically modified by a partial dehydrofluorination followed by anoxidation, per 30 to 99 parts, respectively, of an epoxy primer,

a layer (3) of fluoropolymer.

The coating obtained has good impact strength, flexibility allowingslight bending of the tube and excellent adhesion to the metal, even athigh temperature (up to 150° C.). These good properties are maintainedon contact with seawater.

DETAILED DESCRIPTION OF THE INVENTION

The term “epoxy primer” used for the layer (1) advantageously denotesthe product of the reaction of a thermosetting epoxy resin and of ahardener. Their principle is disclosed, for example, in Kirk-OthmerEncyclopaedia of Chemical Technology Vol. 9—pages 267-289, 3^(rd)edition. This layer (1) may also be defined as any product of thereaction of an oligomer bearing oxirane functions and of a hardener. Asa result of the reactions carried out during the reaction of these epoxyresins, a crosslinked material is obtained corresponding to athree-dimensional network which is more or less dense depending on thebase characteristics of the resins and hardeners used.

The term “epoxy resin” means any organic compound containing at leasttwo functions of oxirane type, which can be polymerized by ring-opening.The term “epoxy resin” denotes any common epoxy resin which is liquid atroom temperature (23° C.) or at higher temperature. These epoxy resinsmay be monomeric or polymeric, on the one hand, and aliphatic,cycloaliphatic, heterocylic or aromatic, on the other hand. Examples ofsuch epoxy resins which may be mentioned include resorcinyl diglycidylether, bisphenol A diglycidyl ether, triglycidyl p-aminophenol,bromobisphenol F diglycidyl ether, m-aminophenyl triglycidyl ether,tetraglycidylmethylenedianiline, (trihydroxyphenyl)methane triglycidylether, polyglycidyl ethers of novolac phenol-formaldenhyde, polyglycidylethers of novolac ortho-cresol and tetraglycidyl ethers oftetraphenylethane. Mixtures of at least two of these resins may also beused.

The preferred resins are epoxy resins containing at least 1.5 oxiranefunctions per molecule and more particularly epoxy resins containingbetween 2 and 4 oxirane functions per molecule. Epoxy resins containingat least one aromatic ring, such as bisphenol A diglycidyl ethers, arealso preferred.

As regards the hardener, the hardeners generally used are epoxy-resinhardeners which react at room temperature or at temperatures above roomtemperature. Non-limiting examples which may be mentioned include:

Acid anhydrides, including succinic anhydride,

Aromatic or aliphatic polyamines, including diaminodiphenyl sulphone(DDS) or methylenedianiline or4,4′-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA),

Dicyandiamide and its derivatives,

Imidazoles,

Polycarboxylic acids,

Polyphenols.

The resins used in the present invention are crosslinkable between 180°C. and 250° C.

The gel time is defined by AFNOR standard NFA 49-706. This is the timerequired to bring about a rapid increase in viscosity at a giventemperature. The gel time is advantageously between 20 and 60 seconds.

The Tg is advantageously greater than 120° C. These resins may be in theform of powder or liquid which is sprayed onto the metal surface, whichhas been degreased, sanded and heated beforehand.

These are advantageously one-component powder resins which areconventionally obtained as follows:

The epoxy resin (solid at room temperature, e.g.: high mass DGEBA), thehardener, optionally the accelerators, the fillers, etc. are mixedtogether in the molten state; during this step, pre-crosslinking takesplace but without going as far as the gel point,

The mixture is cooled at the mixer outlet, so as to stop thecrosslinking,

The homogeneous solid obtained is made into powder.

A one-component powder is thus obtained which can be applied by theusual processes and which completes its crosslinking on contact with hotmetal. Generally, for these applications, the preferred systems arethose which crosslink only at high temperature, (180-240° C.) such thatthere is no problem of storage at room temperature (shelf life or potlife of 6 months-1 year).

These resins may comprise additives such as silicones, pigments such astitanium dioxide, iron oxides, carbon black, fillers such as calciumcarbonate, talc or mica.

As regards the acrylic polymer (L1) of the layer (2), it consistsessentially of alkyl (meth)acrylate units. The other monomersconstituting (L1) may be acrylic or non-acrylic monomers, which may bereactive or unreactive. The term “reactive monomer” means: a chemicalgroup capable of reacting with the oxirane functions of epoxy moleculesor with the chemical groups of the hardener. Non-limiting examples ofreactive functions which may be mentioned include: oxirane functions,amine functions, carboxyl functions, acid chloride functions and alcoholfunctions. The reactive monomer may be (meth)acrylic acid or any otherhydrolyzable monomer leading to these acids. Among the other monomerswhich may constitute (L1), non-limiting examples which may be mentionedinclude glycidyl methacrylate and tert-butyl methacrylate. Examples ofpolymer (L1) which may be mentioned include homopolymers of an alkyl(meth)acrylate. Alkyl (meth)acrylates are described in Kirk-Othmer,Encyclopaedia of chemical technology, 4^(th) edition, in Vol. 1, pages292-293 and in Vol. 16, pages 475-478. Mention may also be made ofcopolymers of at least two of these (meth)acrylates and copolymers of atleast one (meth)acrylate with at least one monomer chosen fromacrylonitrile, butadiene, styrene and isoprene, provided that theproportion of (meth)acrylate is at least 50 mol %. (L1) isadvantageously PMMA with a few percent by weight of acid function. Thesepolymers (L1) either consist of the monomers and optionally of thecomonomers mentioned above and do not contain any impact modifier, orthey also contain an acrylic impact modifier. The acrylic impactmodifiers are, for example, random or block copolymers of at least onemonomer chosen from styrene, butadiene and isoprene and of at least onemonomer chosen from acrylonitrile and alkyl (meth)acrylates, and theymay be of core-shell type. The impact modifiers may also be triblockcopolymers consisting of a polystyrene block, a polybutadiene block anda PMMA block. These triblock copolymers are disclosed in patentapplication WO 99/29772. These acrylic impact modifiers may be mixedwith the polymer (L1) once prepared or may be introduced during thepolymerization of (L1) or prepared simultaneously during thepolymerization of (L1). The amount of acrylic impact modifier may be,for example, from 0 to 30 parts per 100 to 70 parts of (L1) andadvantageously from 5 to 20 parts per 95 to 20 parts of (L1). It wouldnot constitute a departure from the context of the invention if (L1) wasa blend of two or more of the above polymers.

The Tg of (L1) is advantageously greater than or equal to 120° C. andpreferably greater than or equal to 130° C.

The MFI (melt flow index) of (L1) may be between 2 and 15 g/10 minmeasured at 230° C. under a 3.8 kg load.

The MVFR (melt volume flow rate) of (L1) may be between 1.5 and 12cm³/10 min measured at 230° C. under a 3.8 kg load. Polymers that aresuitable for (L1) are Sumipex TR® from Sumitomo® and Oroglas HT121® fromAtoglas. These are copolymers of methyl methacrylate and of(meth)acrylic acid.

As regards the fluoropolymer L2 of the layer (2), it is obtained from afluoropolymer which is chemically modified by a partialdehydrofluorination followed by an oxidation. The fluoropolymer which ismodified may be a fluoroplastic or a fluoroelastomer, provided that theycontain units of general formula (I):

in which X and X′ may be, independently of each other, a hydrogen atom,a halogen, in particular fluorine or chlorine, or a perhaloalkyl, inparticular perfluoroalkyl.

The fluoropolymers which may be used may be prepared by polymerizationor copolymerization of olefinic unsaturated monomers. To obtain afluoropolymer having the unit of formula (I), the monomer and/or thecomonomers must comprise both fluorine atoms linked to carbon atoms andhydrogen atoms linked to carbon atoms. For example, the fluoropolymerswhich may be used may be homopolymers prepared from hydrofluorocarbonmonomers, or may be copolymers derived from perfluoro unsaturatedmonomers copolymerized with one or more unsaturated monomers containinghydrogen —H, namely a hydrofluorocarbon monomer and/or a non-fluoromonomer.

As examples of olefinic unsaturated monomers which may be used, mentionmay be made of hexafluoropropylene (HFP), tetrafluoroethylene (TFE),vinylidene fluoride (VF₂), chlorotrifluoroethylene (CTFE),2-chloropenta-fluoropropene, perfluoroalkyl vinyl ethers such asCF₃—O—CF═CF₂ or CF₃—CF₂—O—CF═CF₂, 1-hydropentafluoropropene,2-hydropentafluoropropene, dichloro-difluoroethylene, trifluoroethylene,1,1-dichlorofluoroethylene and perfluoro-1,3-dioxoles such as thosedisclosed in U.S. Pat. No. 4,558,142, and olefinic unsaturated monomerscomprising no fluorine, such as ethylene, propylene, butylene and higherhomologues.

Diolefins containing fluorine may be used, for example diolefins such asperfluorodiallyl ether and perfluoro-1,3-butadiene.

The olefinic unsaturated monomers or comonomers may be polymerized toobtain a fluoropolymer by the processes known in the prior art offluoropolymers.

In particular, as regards the processes for synthesizing poly(vinylidenefluoride) (PVDF), patents U.S. Pat. No. 3,553,185 and EP-A-0 120 524disclose processes for synthesizing PVDF by placing vinylidene fluoride(VF₂) in aqueous suspension and polymerizing it. Patents U.S. Pat. No.4,025,709, U.S. Pat. No. 4,569,978, U.S. Pat. No. 4,360,652, and EP-A-0655 468 disclose processes for synthesizing PVDF by placing VF₂ inaqueous emulsion and polymerizing it.

In general, the olefinic unsaturated fluoro monomers may be polymerizedand optionally copolymerized with non-fluoro olefinic monomers inaqueous emulsions. The emulsions contain, for example, a water-solubleinitiator such as an ammonium or alkali metal persulfate oralternatively an alkali metal permanganate, which produce free radicals,and also contain one or more emulsifiers such as ammonium or alkalimetal salts of a perfluorooctanoic acid.

Other aqueous colloidal suspension processes use initiators that areessentially soluble in the organic phase, such as dialkyl peroxides,alkyl hydroperoxides, dialkyl peroxydicarbonates or azoperoxides, theinitiator being associated with colloids such as methylcelluloses,methylhydroxy-propylcelluloses, methylpropylcelluloses andmethylhydroxyethylcelluloses.

Many fluoropolymers and copolymers are commercially available, inparticular those from the company Elf Atochem S.A. under the brand nameKynar®.

The fluoropolymer which is modified to convert it into L2 is preferablyin the form of an aqueous dispersion, such as an emulsion or asuspension. This dispersion may be the product resulting from one of thesynthetic methods recalled above.

The polymer which is modified to convert it into L2 is preferably PVDFhomopolymer or a VF2-HFP copolymer.

This fluoropolymer is subjected to a partial dehydrofluorination with abase and the fluoropolymer thus partially dehydrofluorinated is thenreacted with an oxidizing agent to give a new fluoropolymer L2.

This dehydrofluorination of the fluoropolymer is obtained by a base inaqueous medium or in an organic solvent. Bases which may be used arecited in WO 98/08880. They may be, for example, a hydroxide such aspotassium hydroxide (KOH), ammonium hydroxide (NH₄OH), sodium hydroxide(NaOH), lithium hydroxide (LiOH), a carbonate such as potassiumcarbonate (K₂CO₃) or sodium carbonate (Na₂CO₃), a tertiary amine, atetraalkylammonium hydroxide or a metal alkoxide. A process for thedehydrofluorination in aqueous medium of a fluoropolymer emulsion isalso disclosed in patent application WO 98/08879. The base may be usedwith or without catalyst. The base may also be an amine derivative ofhydrocarbon-based structure which is soluble or partially soluble inwater or organic solvents, in particular1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or1,4-diazabicyclo[2.2.2]octane (DABCO).

The catalyst may be, for example, tetrabutylammonium bromide (TBAB) andtetraalkylphosphonium, alkylarylphosphonium, alkylammonium andalkylphosphonium halides. The basic compound and the optional catalystmay be dissolved or diluted in a solvent such as naphthalene,tetrahydrofuran (THF) and water.

The oxidation is preferably obtained in heterogeneous aqueous mediumwith hydrogen peroxide (H₂O₂) or with the hypochlorite anion (ClO⁻)following the introduction of a hypochlorous acid salt (ClOM1) in whichthe cation (M1) corresponds, for example, to an alkali metal such assodium or potassium. Specifically, hydrogen peroxide in aqueous phasemakes it possible to have an advantageous process by minimizing thewaste compared with a process using an organic solvent. Hydrogenperoxide in aqueous phase also allows a simplified treatment of theeffluents compared with other oxidizing agents. Hypochlorous acid saltsshow the same advantages as hydrogen peroxide on account of theirpossibility of use in aqueous phase, and have a particular advantage inprocess terms since they can be introduced, partially or totally, fromthe first dehydrofluorination step.

The treatment with hydrogen peroxide may be activated in the presence ofa metal catalyst, such as iron (II), which is introduced, for example,in the form of halides. According to the same principle, the treatmentwith the hypochlorous acid salt may be performed in the presence of ametal catalyst, such as manganese (III) or nickel (II) combined withvarious ligands such as alkylated polyamines, phthalocyanins orporphyrins. The reaction with the hypochlorite anion can also bepromoted by adding an aprotic solvent, such as acetonitrile or glymes.

However, other oxidizing agents that are active in aqueous medium may beused, for example palladium or chromium halides, in particular PdCl₂ andCrCl₂, alkali metal permanganates, for example KMnO₄, peracid compounds,alkyl peroxides or persulfates, optionally combined with H₂O₂ or withactivating co-reagents, such as sodium metabisulphite or potassiummetabisulphite.

The reaction or the contact with aqueous H₂O₂ is advantageouslyperformed at a pH ranging from 6.5 to 8 and preferably from 6.7 to 7.6.The reason for this is that for a pH of less than 6.5, the reaction isvery slow, and for a pH of greater than 8, there is a risk of the H₂O₂decomposition reaction running out of control.

The reaction or the contact with H₂O₂ is advantageously performed at atemperature ranging from 20° C. to 100° C. and better still from 50° C.to 90° C.

The total amount of H₂O₂ added, calculated on the basis of the pureperoxide, is advantageously from 1% to 50% by weight relative to thetotal weight of the reaction medium. This amount preferably ranges from2% to 12%.

The reaction or the contact with the hypochlorite anion isadvantageously performed at a pH ranging from 6 to 14.

The reaction or the contact with the hypochlorite anion isadvantageously performed at a temperature ranging from 20 to 100° C. orbetter still from 50 to 90° C.

The total amount of hypochlorite anion added, calculated on the basis ofsodium hypochlorite (NaClO), is advantageously from 0.1% to 50% byweight relative to the total weight of the reaction medium. This amountpreferably ranges from 0.5% to 10%.

According to the process of the present invention, the modified polymersL2 have adhesion properties that are greatly increased compared withfluoropolymers that are not chemically modified.

The MFI (melt flow index) of L2 is advantageously between 0.2 and 5 g/10min (at 230° C. under a 10 kg load) for L2 derived from the PVDFhomopolymer and between 2 and 10 g/10 min (at 230° C. under a 5 kg load)for L2 derived from the copolymer of VF2 and of HFP.

As regards the fluoropolymer L3 of the layer (2), it may be chosen frompolymers and copolymers containing units of general formula (I) citedabove for the polymers treated to produce L2.

Examples of fluoropolymers L3 which will be cited most particularly are

PVDFs, vinylidene fluoride (VF2) homopolymers and copolymers ofvinylidene fluoride (VF2) preferably containing at least 50% by weightof VF2 and at least one other fluoromonomer such aschlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP),trifluoroethylene (VF3) or tetrafluoroethylene (TFE),

trifluoroethylene (VF3) homopolymers and copolymers,

copolymers, and in particular terpolymers, combining the residues ofchlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE),hexafluoropropylene (HFP) and/or ethylene units and optionally VF2and/or VF3 units.

Among these fluoropolymers L3 that are advantageously used are PVDFhomopolymer and VF2-HFP copolymers.

The melting point of L3 is advantageously greater than 150° C. A meltingpoint which is as high as possible is preferred.

The MVFR (melt volume flow ratio) of L3 is advantageously between 2 and25 cm³/10 min and preferably between 4 and 10 cm³/10 min (at 230° C.under a 5 kg load).

The proportion of L3 is advantageously from 95 to 70 parts by weight per5 to 30 parts, respectively, of at least one polymer chosen from acrylicpolymers L1 and the polymers L2.

The binder of this layer (2) can contain additives and fillers usuallyused in fluoropolymers.

A catalyst capable of increasing the reactivity of the reactivefunctions of L1 or L2 with the epoxy may be added. This catalyst may be1,4-diazabicyclo[2.2.2]octane (DABCO) or methyl-2-imidazole (M2ID).These catalysts are disclosed in French patent FR-A-2 745 733.

A blend of L1 and L2 may also be used, i.e. the binder comprises 98 to50 parts of L3 per 2 to 50 parts, respectively, of a blend of L1 and L2.

The proportion of L3 is advantageously from 95 to 70 parts by weight per5 to 30 parts, respectively, of a blend of L1 and L2.

The binder may be manufactured by mixing together, in molten form, thevarious constituents in the usual devices for blending thermoplasticmaterials, and then used immediately or recovered after cooling in theform of powder or granules. It may also be manufactured by dry-blendingof the various constituents in the form of powder or granules.

As regards the fluoropolymer of the layer (3), it may be chosen from thefamily of fluoropolymers described for L3. It is advantageously a PVDFhomopolymer or a VF2-HFP copolymer having a melting point of at least165° C. The MVFR is advantageously between 0.5 and 5 cm³/10 min (at 230°C. under a 5 kg load).

The fluoropolymer of this layer (3) may contain additives and fillersusually used in fluoropolymers.

A common PVDF plasticizer, a dye or an acrylic impact modifier ortriblock copolymer which may be chosen from the impact modifiers of L1described above may also be added. The proportion of impact modifier inthis layer (3) may be from 5 to 15 parts by weight per 95 to 85 parts,respectively, of fluoropolymer.

The thicknesses of the various layers may be from 50 to 150 μm for thelayer (1), from 100 to 500 μm for the layer (2) and from 1000 to 5000 μmfor the layer (3). The thicknesses are preferably, in μm, starting withthe layer (1): 80/250/1500.

The metal surface may be of any type; however, the invention isparticularly useful for the outer surface of tubes, these tubes possiblyhaving an outside diameter, for example, of up to 0.8 or 1.5 m and athickness of from 2 to 25 mm.

As regards the coating of tubes, the preferred process breaks down asfollows:

preparation of the steel tube: degreasing, shot-blasting and optionallysurface treatment (chromatation, etc.);

treatment of the tube in an infrared oven so as to reach a temperatureof between 180 and 220° C.;

application of the epoxy primer powder by electrostatic spraying (orother process in the case of a liquid epoxy resin);

application of the binder by lateral extrusion, with bearing from a rollpress. The time separating the application of the epoxy resin and theapplication of the binder must be less than the gel time of the epoxy;

application of the outer layer made of fluoropolymer by lateralextrusion, bearing from a roll press;

cooling with water.

The binder and the outer layer may also be extruded using a “crosshead”surrounding the tube. The binder and optionally the outer layer may alsobe applied by a powder process.

Besides the degreasing and shot-blasting, the steel may be chromated orsilanized to improve the attachment of the primer.

As regards the first variant, the thicknesses of the various layers maybe from 50 to 150 μm for the layer (1) and from 1 000 to 5 000 μm forthe layer (2). The thicknesses are preferably, in μm, starting from thelayer (1): 80/1 500. All the other elements described for the maininvention apply.

As regards the second variant, the thicknesses of the various layers maybe between 100 and 500 μm for the layer (2) and from 1 000 to 5 000 μmfor the layer (3). The thicknesses are preferably, in μm, starting fromthe layer (2): 250/1500. All the other elements described for the maininvention apply.

As regards the third variant, the thicknesses of the various layers maybe from 100 to 500 μm for the layer (1) and from 1 000 to 5 000 μm forthe layer (3). The thicknesses are preferably, in μm, starting with thelayer (1): 250/1 500. The primer is advantageously prepared bydry-blending the one-component epoxy resin powder and the L2 powder. Allthe other elements described for the main invention apply.

EXAMPLES

Materials Used

Steel Tube:

Welded steel tube (shade E36-4) of length 3 meters, outside diameter114.3 mm and thickness 6.3 mm, sold by Van Leeuwen Tubes (45120 Chalettesur Loing, France).

Chromatation:

Accomet PC chromatation system, sold by Brent Europe Ltd. (address:Ridgeway, Iver, Buckinghamshire, SLO 9JJ, UK).

Epoxy Primers of the Layer (1):

Eurokote® 798: Epoxy primer powder produced by BS Coating. Gel time 45±5s at 180° C. Tg=120-140° C. (DSC on crosslinked film).

Scotchkote® 6258: Novolac epoxy primer powder produced by 3M®. Gel time26 s at 182° C. Tg=166° C. (DMA on crosslinked film).

Materials Used for the Binders of the Layer (2):

Polymers L3:

Kynar® 3120-15: HFP/VF2 copolymer produced by Atofina, with a meltvolume flow rate MVFR=4 cm³/10 min at 230° C. under 5 kg, and a meltingpoint of 165° C.

Kynar® 2850-04: HFP/VF2 copolymer produced by Atofina, with a meltvolume flow rate MVFR=10 cm³/10 min at 230° C. under 5 kg, and a meltingpoint of 158° C.

Acrylic Polymer L1:

Oroglas® HT121: copolymer of methyl methacrylate and of acrylic acid,produced by Atofina, of Tg=130° C. and MVFR=1.8 cm³/10 min at 230° C.under 3.8 kg.

Fluoropolymer L2:

MKB212: Product obtained according to the procedure described later,starting with a PVDF latex which is a precursor of Kynar® 1000HD, PVDFhomopolymer of MVFR=1.1 cm³/10 min at 230° C. under 5 kg and with amelting point of 169° C. The melting point of MKB212 is 168° C.

Fluoropolymer of the Layer (3):

Kynar® 740: vinylidene fluoride homopolymer produced by Atofina, with amelt volume flow rate MVFR=1.1 cm³/10 min at 230° C. under 5 kg and amelting point of 168° C.

Preparation of MKB212:

A polyvinylidene fluoride latex Kynar 1000 HD, prepared according to theemulsion process as disclosed in patent U.S. Pat. No. 4,025,709, is usedas fluoropolymer starting material. After drying at 105° C. for 24hours, this latex gives a dry powder. This latex, referred to as Latex 1hereinbelow, contains 40% by weight of PVDF. However, the processaccording to the present invention may be applied in particular to anyPVDF latex or VF2 copolymer obtained by an emulsion process or to anysuspension of PVDF or VF2 copolymer obtained by a suspension process.

Dehydrofluorination step: The preparation of 7.2 kg of an aqueous sodiumchloride solution containing 15% by weight of NaOH in water is commencedin a stirred 20-liter reactor. This solution is brought to 70° C.,followed by addition thereto of 7.2 kg of Latex 1 optionally diluted indeionized water so as to have a given solids content, at a rate of 0.72kg/min with stirring at 180 rpm. A brown coagulated emulsion is thusobtained which becomes progressively darker the more the degradationproceeds. Depending on the dehydrofluorination reaction time, a fineblack powder is obtained which becomes progressively insoluble in theusual organic solvents, in particular dimethylformamide (DMF) orN-methylpyrrolidone (NMP).

Step of reaction with an oxidizing agent: The reaction medium isacidified to pH=5, with continuous stirring and while maintained at atemperature of 70° C., by adding about 2.5 kg of 36% by weighthydrochloric acid. 1.68 kg of 35% by weight hydrogen peroxide are thenadded at a rate of 0.4 kg/min, after which the pH is increased to avalue of between 6.6 and 7.6 by adding a sodium hydroxide solutioncontaining 15% by weight of NaOH. The mixture is left to react whilemaintaining the pH between 6.6 and 7.6 by adding the same sodiumhydroxide solution. A gradual decoloration of the coagulated emulsion isobserved, which becomes pale yellow to ochre.

Finishing: The solid coagulate in suspension is filtered to give a paleyellow powder which is washed with three dispersions in 20 liters ofwater with stirring and successive filtrations. A powder is thusobtained which is dried in an oven at 105° C. to constant weight.

Characterization: The characterization of this powder is carried out bymeasuring the absorbance at 300 nm which is obtained by analysis with aPerkin-Elmer LC-75 spectrophotometer using a concentration of 0.1% byweight of product in NMP. The dissolution time is 24 hours beforecarrying out the measurement.

Structures with Binder Comprising the Acrylic Polymer L1

Example 1

Kynar 3120-15 and Oroglas HT121 are mixed together in a Fairex Super2/50 single-screw extruder in a proportion of 85/15 by mass. The mixtureis granulated at the extruder outlet.

The steel tube to be coated (114 mm outside diameter) is degreased andthen shot-blasted. Immediately after this operation, the tube mounted ona gantry support rotating at 10 rpm and advancing at 50 cm/min is heatedto 200° C. with an induction oven and coated with the primer powderEurokote 798 sprayed via a spray gun. The laterally extruded binder iswound around the tube on the primer 10-20 s after depositing thisprimer. The Kynar 740 also extruded laterally, coats the first twolayers immediately after. A roll press ensures good contact between thevarious layers. The coated tube is cooled with water for 3 minutes.

The flow rates of the spray gun and of the two extruders are adjusted soas to have 70-100 μm of primer, 250-350 μm of binder and 1 250-1 500 μmof Kynar 740.

In summary, the structure of the coating obtained is as follows:

Eurokote 798/binder {85% Kynar 3120-15, 15% Oroglas HT121}/Kynar 740 80μm/300 μm/1 400 μm

For the assessment, the tube is cut into rings which are then assessedby peeling according to standard prEN 10 285:1 998. For each temperaturetest (110, 130 and 150° C.), three rings are peeled. The mean force andthe standard deviation are calculated on the three values obtained. Thepeeling results and the breaking modes are given in Table I.

Example 2

The structure below is prepared in the same way as in Example 1:

Eurokote 798/binder {70% Kynar 3120-15, 30% Oroglas HT121}/Kynar 740 80μm/300 μm/1 400 μm

Example 3

The structure below is prepared in the same way as in Example 1, but ona chromated tube:

Accomet PC/Eurokote 798/binder {85% Kynar 3120-15, 15% OroglasHT121}/Kynar 740

80 μm/300 μm/1 400 μm

The Accomet PC chromating solution is applied by brush to the tube aftershot-blasting. The treatment in the induction oven at 200° C. sufficesto ensure good drying before applying the primer.

Example 4

The structure below is prepared in the same way as in Example 3, butwith a thicker outer layer:

Accomet PC/Eurokote 798/binder {85% Kynar 3120-15, 15% OroglasHT121}/Kynar 740

80 μm/300 μm/2 000 μm

Example 5

The structure below is prepared in the same way as in Example 3:

Accomet PC/Scotchkote 6258/binder {85% Kynar 2850-04, 15% OroglasHT121}/Kynar 740

80 μm/300 μm/1 400 μm

Example 6

The structure below is prepared in the same way as in Example 1:

Eurokote 798/binder {85% Kynar 2850-04, 15% Oroglas HT121}/Kynar 740

80 μm/300 μm/1 400 μm

TABLE I Results of peeling in N/cm and breaking mode at 110, 130 and150° C. Structure Example 1 Example 2 Example 3 Example 4 Example 5Example 6 110° C. Mean 147 134 141 158 167 164 Standard ±9 ±8 ±5 ±10 ±4±10 deviation Mode CP CP C C CP + F CP + F 130° C. Mean 108 114 117 106125 — Standard ±9 ±10 ±10 ±14 ±1 — deviation Mode CP CE C C CP + F —150° C. Mean 40 7 56 42 40 10 Standard ±10 ±16 ±21 ±1 ±3 deviation ModeCP/AO AO C C C AO Breaking modes: C = cohesive break in the binder, CE =cohesive break in the binder in the outer layer interface region, CP =cohesive break in the binder in the epoxy primer layer interface region,AO = primer adhesive-metal break, +F = with creep of the peeling arm

Structures with Binder Comprising the Fluoropolymer L2:

Example 8

The structure below is prepared in the same way as in Example 1:

Eurokote 798/binder {85% Kynar 3120-15, 15% MKB212}/Kynar 740

80 μm/300 μm/1 400 μm

The results are given in Table II.

Example 9

The structure below is prepared in the same way as in Example 1:

Eurokote 798/binder {70% Kynar 3120-15, 30% MKB212}/Kynar 740

80 μm/300 μm/1 400 μm

Example 10

The structure below is prepared in the same way as in Example 1:

Scotchkote 6258/binder {85% Kynar 3120-15, 15% MKB212}/Kynar 740

80 μm/300 μm/1 400 μm

TABLE II Results of peeling in N/cm and breaking mode at 110, 130 and150° C. Structure Example 8 Example 9 Example 10 110° C. Mean 169 187162 Standard deviation ±4 ±2 ±1 Mode CE + F CE + F C 130° C. Mean 134142 128 Standard deviation ±4 ±5 ±2 Mode CE + F CE + F CE + F 150° C.Mean 10 11 90 Standard deviation ±1 ±5 Mode AO AO CE + F Breaking modes:C = cohesive break in the binder, CE = cohesive break in the binder inthe outer layer interface region, CP = cohesive break in the binder inthe epoxy primer layer interface region, AO = primer adhesive-metalbreak, +F = with creep of the peeling arm

What is claimed is:
 1. A coated metal surface comprising, successively:a layer (1) of epoxy primer placed next to the metal, a layer (2) ofbinder comprising a blend comprising 98 to 50 parts by weight of atleast one fluoropolymer L3 and 2 to 50 parts of at least one polymerchosen from polymers L2 which are fluoropolymers chemically modified bya partial dehydrofluorination followed by an oxidation, a layer (3) offluoropolymer.
 2. Coated A coated metal surface comprising,successively: a layer (1) of epoxy primer placed next to the metal, alayer (2) of binder comprising a blend comprising 98 to 50 parts byweight of at least one fluoropolymer L3 and 2 to 50 parts of at leastone polymer chosen polymers L2 which are fluoropolymers chemicallymodified by a partial dehydrofluorination followed by an oxidation.
 3. Acoated metal surface which is the outer surface of tubes, comprising,successively: a layer (2) of binder placed next to the metal andcomprising 98 to 50 parts by weight of at least one fluoropolymer L3 per2 to 50 parts, respectively, of a mixture comprising at least onepolymer chosen from the polymers L2 which are fluoropolymers chemicallymodified by a partial dehydrofluorination followed by an oxidation, andoptionally at least one polymer chosen from acrylic polymers L1, a layer(3) of fluoropolymer.
 4. A coated metal surface comprising,successively: a layer (1) of primer placed next to the metal andcomprising 1 to 70 parts of a polymer chosen from polymers L2 which arefluoropolymers chemically modified by a partial dehydrofluorinationfollowed by an oxidation, per 30 to 99 parts, respectively, of an epoxyprimer, a layer (3) of fluoropolymer.
 5. A coated metal surfaceaccording to claim 1, in which the epoxy primer is the product of thereaction of a thermosetting epoxy resin and of a hardener.
 6. Coatedmetal surface according to claim 5, in which the gel time defined byAfnor standard NFA 49-706 is between 20 and 60 seconds.
 7. A coatedmetal surface according to claim 5, in which the epoxy primer has a Tggreater than 120° C.
 8. A coated metal surface according to claim 1,containing an acrylic polymer L1 which is a copolymer of methylmethacrylate and of acrylic acid.
 9. A coated metal surface according toclaim 1, containing an acrylic polymer L1 having a Tg greater than orequal to 120° C.
 10. A coated metal surface according to claim 1,wherein the chemically modified fluoropolymers are chemically modifiedto obtain a fluoroplastic of or a fluoroelastomer which contains unitsof general formula (I):

in which X and X′ may be, independently of each other, a hydrogen atom,or a halogen, or a perhaloalkyl.
 11. Coated metal surface according toclaim 10, in which the oxidation to prepare L2 is obtained inheterogeneous aqueous medium with hydrogen peroxide (H₂O₂) or with thehypochlorite anion (CIO).
 12. A coated metal surface according to claim1, in which the fluoropolymer L3 is polyvinylidene fluoride (PVDF)homopolymer or a vinylidene fluoride-hexafluoropronviene (VF2-HFP)copolymer.
 13. A coated metal surface according to claim 1, in which themelting point of L3 is greater than 150° C.
 14. A coated metal surfaceaccording to claim 3, in which the fluoropolymer of the layer (3) ispolyvinylidene fluoride (PVDF) homopolymer or a vinylidenefluoride-hexafluoropropylene (VF2-HFP) copolymer having a melting pointof at least 165° C.
 15. A coated metal surface according to claim 1, inwhich the surface is an outer surface of a tube.
 16. A coated metalsurface according to claim 10, wherein said at least one X and X′ ischlorine, fluorine or perfluoroalkyl.
 17. A coated metal surfaceaccording to claim 1, wherein the metal is steel.
 18. A coated metalsurface according to claim 15, wherein the metal is steel.
 19. In amethod of transporting oil through a tube, the improvement wherein thetube is comprises a coated metal surface in accordance with claim 18.20. A method according to claim 19, wherein the tube is in sea water andthe oil is hot oil.
 21. A coated metal surface according to claim 2, inwhich the fluoropolymer L3 is polyvinylidene fluoride (PVDF) homopolymeror a vinylidene fluoride-hexafluoropropylene (VF2-HFP) copolymer.
 22. Acoated metal surface according to claim 4, in which the fluoropolymer L3is polyvinylidene fluoride (PVDF) homopolymer or a vinylidenefluoride-hexafluoropropylene (VF2-HFP) copolymer.
 23. A coated metalsurface according to claim 10, in which the fluoropolymer L3 ispolyvinylidene fluoride (PVDF) homopolymer or a vinylidenefluoride-hexafluoropropylene (VF2-HFP) copolymer.